I'm thinking that 8/16-Bit boards be obsolete 32-Bit boards require less energy to run with a smaller footprint.

Then again the Arm based Due doesn't have very powerful output pins and are not 5 volt tolerant for input pins, so many projects based on the Due may end up needing more external components then a AVR based arduino board. Also building standalone AVR projects that one developed and debugged on a AVR arduino board is a piece of cake costing only less the $10 worth of parts, making a standalone ARM/Due based board may well be beyond the capabilities of many hobbyist. So it really comes down to the specifics of the project at hand, and the best world is where one has a choice rather then having to adapt. So I think there is room for 8 bit AVR based boards for quite while yet.

I personally don't have a need for that much processing power, nor the time to write enough software to take advantage of it.

I guess that you never programmed something like a small TFT Display (320x240 pixel). Such a simple element like a progressbar brings any ATMega MPU on the border of usage. Atmel's xMega's can help to solve those problems because they clocked 30Mhz but the progressbar is not the only thing that the mpu should doing. To use bigger Fonts need do define them as a hugh uint8_t array an this consumes storage space in the Flash memory. An access to that memory from a running program, consumes many cpu time. This is reasoned by the modified harvard-architecture design of the ATMega's, that the compiler not supports. GNU-CC is designed for used with a von-Neumann architecture. To get access to the flash memory, the compiler designer had created some macros that will solve it. I am very surprised about the speed of a simple Cortex-M3 that is internaly clocked with 78Mhz. The size of flush memory is 256kbyte (STM32V103C) and the SRAM size is 48Kbyte. This chip price is cheaper than an ATMega 2560 and the tools are cheaper too. The backside of the medal is, that the architecture is more complex. I guess, that the ARM Cortex M3 / M4F (with DSP - every floating point application will love it) will be very successful like ATMega and PIC's. I have a couple of applications that the power of the ARM is required. I will be with you, i also know a couple of applications that does'nt need a "big gun" like an ARM. ATMega's MPU's are are very versatile in usage and is available as a DIP variant's. This are very important for test and evaluation purposes. I don't will miss them and will use them in the future too.

Just remember all those graphics program's that were written on 8 bit micros running at a quarter the speed of an arduino that were produced in the 80s.

Brings back fond memories of the C64/C128 and the amazing work people did in assembly to make that processor sing, speak, and display some pretty amazing graphics.

As for whether 8-bit CPUs are obsolete, there is a place for them, so why not use them? As others have pointed out, the ability to drive strong signals without having to resort to external transistors is a good one. But is it life-changing? No... just that especially for beginners the ability to interface directly with an MCU is a big help because it potentially eliminates a link or two in a chain of things that can go wrong.

8-bit microprocessors will remain for as long as manufacturers are willing to make them. My guess is that the low end of the market is slowly eroding as manufacturers upgrade their manufacturing lines and obsolete old MCUs in the process. As an example, look at the DS2423, a chip with a dedicated following - discontinued with no substitute. By eliminating that chip Dallas/Maxim also guaranteed the death of the 1-Wire platform for many other applications - counters are very useful!

Margin pressure will also play into this, it's pretty funny to see 8-bit MCUs selling for more $$$ than computationally much more powerful 32-bit MCUs. At some point, the market will hit the tipping point and the bulk of development will go to 32-bits even if a 8-bit MCU was perfectly adequate.